This invention relates to a device for fixing fuel assemblies to a nuclear reactor core in a reactor pressure vessel of a boiling water reactor (BWR) for preventing fuel assemblies from upwardly rising and safely inserting a control rod into spaces between channel boxes of the fuel assemblies to promptly shut down the BWR on an occurrence of an emergency such as earthquakes.
FIG. 1 shows a fuel assembly unit of a prior type inserted into an upper grid 20 secured to a reactor core, not shown. The unit generally comprises four fuel assemblies 1 as shown in FIG. 2. Each of fuel assemblies 1 is supported at its lower end by inserting the lower end into a hole provided on a fuel support 23 and at its upper end by the upper grid 20 and channel fasteners 22 which firmly interconnects the respective fuel assemblies 1 thereby to control or limit the horizontal movement or vibration of the fuel assemblies. In a BWR, though the irregular horizontal movement or vibration of the fuel assemblies is thus limited or suppressed, there is no specific means for limiting or suppressing vertical movement or vibration of the fuel assemblies.
During the operation of a BWR, when an emergency such as large earthquake occurs, it is well known to insert a control rod having a cross shaped cross-section between the spaces defined by the channel boxes of the fuel assemblies as shown in FIG. 2 (a top plan view) to rapidly shut down the operation. However, in such case, the fuel assemblies move horizontally in an accelerated manner and flex or bend at their longitudinal central portions. These flexed or bent portions contact with the control rods now being inserted at the time of an emergency, thereby to generate a force for upwardly pushing the fuel assemblies 1, and this upward force is added to the vertical vibration of the fuel assemblies 1 which is caused by the earthquake. The upward force thus amplified may sometimes move upwardly the fuel assemblies 1 to a mechanically unstable position and may be damaged by the subsequent drop of the raised fuel assemblies. Therefore, in order to ensure the insertion of the control rod into the fuel assembly unit thereby to control the operation of a nuclear reactor, it becomes necessary to safely limit or suppress the vertical movement of the fuel assemblies.